In the field of centrifugal compressors, the attachment and removal of compressor impellers from compressor shafts is of continuing concern. The mounts must be secure enough to hold the impeller onto the compressor shaft, yet must also allow the compressor impeller to be relatively easily removed. One method of coupling an impeller to an impeller shaft is to provide the impeller with a longitudinally tapered cylindrical hole to mate with a similarly tapered portion of the compressor shaft. In this arrangement the compressor shaft typically has a threaded end portion adjacent to a tapered portion. The impeller is placed over the threaded portion on the end of the shaft until the tapered portion of the impeller engages the tapered portion of the shaft, leaving the threaded portion exposed. A lock nut is then screwed onto the threaded portion of the shaft and tightened, forcing the impeller taper firmly against the shaft taper, securing them together. This mounting method is often sufficient for low speed and low torque compressor applications. Unfortunately, a tapered fit such as this cannot counteract significant torsional loads. As the speed and the torque of a compressor increase, the tapered portion of the impeller will spin on the shaft. The advantage of the tapered fit is the ease of removal. An impeller with a tapered fit need only be moved axially about 25 inches to release its frictional grip on the shaft. When an impeller having a cylindrical bore is fitted onto a cylindrical portion of a shaft, the impeller must be press fit over the entire length of the cylindrical mating surfaces, typically 5-10 inches. This may require heating the impeller so it can be easily slid this 5-10 inches.
One method of preventing this spin is to mount the impeller hydraulically on the shaft. In this method, hydraulic fluid under pressure is injected axially into the shaft and then is conducted radially outward between the tapered impeller surface and the tapered shaft surface as the impeller is installed. The hydraulic fluid provides two benefits. First, it reduces the friction between the impeller and the shaft as the impeller is installed and second, the fluid pressure between the tapered surfaces expands the tapered bore of the impeller allowing it to be pushed even farther onto the tapered portion of the shaft. Once the impeller is in the proper position, the hydraulic pressure is released and the impeller taper contracts to fix it more firmly onto the compressor shaft than would otherwise be provided. The hydraulic fit also has the advantage of eliminating the use of torches, heaters or other devices for installation and removal. These implements significantly increase the risk of fire and explosions in chemical plant environments.
An alternative method for fixing the impeller on the shaft is to key the tapered surfaces of the impeller and the shaft together. Typically, an oval slot is milled in the compressor shaft taper and a flat key inserted in this slot. A mating keyway is milled into the inner tapered surface of the impeller. To assemble the two, the key is aligned with the keyway slot in the impeller and the impeller is forced onto the compressor shaft. This provides the advantage of the tapered coupling ease of installation as well as the keyed coupling for absorbing torsional stresses. Typically, if the impeller starts to spin with respect to the shaft, it rotates a fraction of a degree at best. The key prevents any further relative rotation and thereby carries the torsional load between the shaft and the impeller. Unfortunately, when a key is employed between the tapered portions of the impeller and the shaft, the hydraulic mounting method cannot be employed due to the irregular surface and leakage paths caused by the keyed construction.
An alternative design incorporating the benefits of the hydraulic mount and the keyed construction has also been employed. In this prior art device, a separate locking ring is placed over the end of the compressor shaft once the impeller is hydraulically installed. This ring, called a "torque plate" is placed over the portion of the compressor shaft that extends completely through the impeller. Once it is placed around the shaft, the lock nut is then screwed onto the end of the compressor shaft to hold the shaft impeller and torque plate together. The torque plate is keyed to fit into both the stub end of the compressor shaft and the impeller, acting as a large external torsional coupling between the two, transmitting torque from the compressor shaft to the impeller. This design has serious limitations, however. Since the torque plate is an additional element disposed between the impeller and the lock nut, the compressor shaft must be lengthened, which increases the overhang of the compressor. Furthermore, the torque plate adds a significant amount of mass to the impeller/shaft assembly. The additional mass further increases the overhang.
What is needed is a new impeller shaft mounting structure that will provide the benefits of a hydraulically mounted fit with the torsional capacity and reduced additional mass of a keyed connection.